Heterodyne Interferometry measures the phase of a continuous signal for as long as the signal remains stable over all portions of a reference path and a measurement path. The major practical problem with high resolution interferometry is that every optical pathlength change in the system, including the ones that are not intended, are measured. After the beams are split in the interferometer, movement of optical components, especially mirrors, add or subtract optical path length from one of the two beams separately, resulting in a signal that is unrelated to the measurement. Therefore, stability of optical components in the separate legs of the interferometer is critical, as described in J. D. Trolinger, Ultra High Resolution Interferometry, Proc. SPIE Vol. 2816, pp114-123 (1996).
Typically, existing heterodyne interferometer devices are subject to thermal drift errors that limit their performance. In addition, they consist of a precision assembly of critical components leading to high cost to manufacture. Therefore, a need exists for a robust, easily manufacturable device, which is immune to thermal drift errors.